Surface irrigation systems in which a network of troughs or ditches is formed in the earth's surface for conveying and distributing water to plants, trees and the like in arid areas or those afflicted with a drought are well known. Also, it is well known to employ underground piping systems for conveying water to outlets or sprinkler heads disposed at or above ground level for discharging or spraying such water on adjacent vegetation. Although these systems are often admirably suited for the purposes for which they were designed, they also have certain disadvantages. For example, in open ditch irrigation water wastage from evaporation and deep seepage in porous soils can be extensive. On the other hand, in non-porous soils water tends to collect and stagnate in pools thereby creating pollution problems and providing breeding areas for mosquitoes and other insects. In the use of sprinkler or spray systems, water is wasted by evaporation, insecticides are often washed off plants, thereby negating their purpose and needlessly contaminating the soil. Also, in some instances, the resultant wet foliage promotes plant diseases.
In view of experience with the above-mentioned systems, it has been found that underground or subsurface irrigation systems offer many advantages over surface irrigation systems. However, subsurface systems have in many instances been impracticable because of the high cost of providing and laying out suitable piping and the expensive accessory equipment such as high pressure pumps, flow regulating devices, and the like, required with such systems. Moreover, their performance has often been unsatisfactory since the water outlet perforations or orifices in these underground pipes become clogged, thus reducing efficiency and making it even more difficult to control properly the rate of flow of water delivered therefrom and to confine the delivered water to the desired area.
Microporous membranes of hydrophilic materials have been used in sub surface irrigation systems. For example, U.S. Pat. No. 3,830,067 to Osborn discloses permeable tubing through which water may pass with negligible pressure drop. U.S. Pat. No. 7,748,930 to Gesser discloses a microporous tubing of a hydrophobic material with a hydrophilic coating.
The present inventors have discovered that microporous membranes of uncoated hydrophobic materials with inherent resistance to water flow have unexpectedly high water flow rates in the presence of plants even at head pressures below their hydrostatic head pressure (hydrohead.) These materials unexpectedly interact with plants to provide a regulated and optimized water or irrigant flow, in particular over longer distances than are otherwise possible.